1. Field of the Invention
This invention relates to a process for separating contaminants from the exhaust gases generated from combustion of fossil fuels and from chemical processes, and more particularly, to the removal of nitrogen and sulfur oxides from a cold stream of gaseous combustion products such as the removal of nitrogen oxides and sulfur oxides from exhaust gases.
2. Description of the Prior Art
Gas recovery devices are well known and in particular, devices associated with power generation plants and coal, oil or gas/fired boilers and chemical processes for removing NO.sub.x and SO.sub.x from the exhaust gas and fugitive gas recovery streams. An example of conventional devices that not only recover heat from flue gases but also remove the contaminants is disclosed in U.S. Pat. No. 4,121,541 where flue gases from a power generating plant are purified and heat is recovered from the flue gases. Flue gases from the boiler initially enter a heat exchanger, where they are cooled and thereafter contacted with cooling water to remove waste heat. Pollutants contained in the flue gases are partially absorbed by cooling water. Thereafter, the purified flue gas is released with ambient cooling air and to the surrounding atmosphere. This system is characteristic of a wet flue gas scrubber used to remove contaminants in the form of sulfur dioxide, fluorine compounds, nitrogen oxides, etc.
U.S. Pat. No. 4,784,835 discloses a gas scrubber and heat exchanger for removing contaminants from sulfurous flue gases, where a glass fiber mat electrostatic filter retains particles within the flue gases. Other known devices for removing contaminants from exhaust gases emitted from power plants are disclosed in U.S. Pat. Nos. 3,473,298; 4,681,744; and 4,799,941. With these devices, the exhaust gases are first chilled with direct water sprays and thereafter solid contaminants and water soluble substances are removed from the gases by the contacting water. In a spray chamber the water combines with water soluble gases, such as SO.sub.x, contained in the gases to form sulfurous and sulfuric acids which are collected with the water spray in a chamber. It is also known as disclosed in U.S. Pat. No. 3,881,004 to recover nitric acid by scrubbing a tail gas with acid or alkaline solution and nitric acids which minimize the discharge of nitrogen oxides to the atmosphere.
While heat recovery devices or economizers are well known, their efficiency needs to be improved because it has been determined that up to 16% of heat escapes uncovered from the stacks equipped with economizers. This in part can be attributed to admitting the exhaust gases at a relatively high exit temperature. Also, in the case of "mass transfer" heat recovery devices the primary function is to recover heat. Generally, the removal of the contaminants from the flue gas stream is incidental. When scrubbing operations are combined with heat removal operations, the process becomes less efficient at heat transfer because a portion of the heat is lost in an effort to remove contaminants.
It is generally recognized that wet scrubbers are designed primarily to remove contaminants and are not efficient in recovering waste heat. Heat removal is considered incidental and adds to the expense of the pollution abatement operation. In many instances after wet scrubbing, flue gas must be reheated to rise up and out of the boiler chimneys. As a result, boilers equipped with gas scrubbing equipment are expensive to maintain and to operate. While it has been suggested by the prior art devices to remove contaminants in the form of NO.sub.x and SO.sub.x emissions from flue gases, the known devices are expensive to operate and waste a substantial amount of energy in the loss of heat that could be otherwise recovered from the flue gases in the process of removing the contaminants from the flue gases.
Therefore, there is need to provide apparatus for removing contaminants, specifically NO.sub.x and SO.sub.x emissions, from exhaust gases that allows for the efficient use of recovered waste heat, while at the same time reducing the content of the contaminants in the exhaust gases to the required levels as prescribed by air quality regulations. Improved emission equipment is therefore required that not only brings the content of the contaminants in the exhaust gases into compliance with regulated air quality standards, but also lowers the cost of the treatment process by recovering heat from the treated exhaust gases to lower the operating cost of the combustion or chemical processing unit generating the exhaust gases.